The supercontinent Pangea as it was 200 million years ago, with black areas representing the extent of a magma flood that lasted several million years.

Hundreds of basalt outcroppings rimming the Atlantic Ocean are actually the remnants of a single huge volcanic eruption that may have triggered mass extinction some 200 million years ago, according to a study led by a Berkeley researcher.

The research team, headed by Paul R. Renne, director of the Berkeley Geochronology Center and adjunct associate professor of geology and geophysics, concluded that the basalt dikes, sills and lavas resulted from the most extensive pulse of magma eruptions known to date.

Sills are what remain after magma intrudes into horizontal underground fissures, then cools and is subsequently exposed. Dikes are the remnants of magma that flowed into vertical fractures. Lava is magma that broke through the Earth's crust and flowed on the surface before cooling.

Remnants of the eruptions are dispersed from the New Jersey Palisades and the Brazilian Amazon to Spain and West Africa. At the time these areas were near one another in the center of a supercontinent known as Pangea. The eruptions began a process that drove the land mass apart to create the Atlantic Ocean.

The time of the eruptions matches within 20,000 years a global extinction at the end of the Triassic period and the beginning of the Jurassic. About half of all marine species, mostly the ammonoid and bivalve mollusks, died out. On land, several families of reptiles disappeared. Paleontologists regard the extinction as one of the most deadly in the 600-million-year history of multi-celled life. Many scientists believe these changes set the stage for the rise of dinosaurs in the Jurassic period.

The close correspondence between the magma flows and the mass extinction suggests that the environmental effects caused by a long series of volcanic eruptions could have set off or at least exacerbated the disappearance of life.

The study suggests that as the magma surfaced in the form of volcanoes over a large area of Pangea, noxious aerosols and greenhouse gases disrupted the global climate.

"This is still one of the most intriguing issues in geology, the relationship between extinctions and cataclysms such as magma floods or asteroid impacts," Renne said. "What we now have is another piece of evidence that shows there was a relationship between flood basalts and biologic crises."

A report detailing the extent of the basalts appears in the April 23 issue of Science. Renne's coauthors include postdoctoral fellow Andrea Marzoli, formerly of the Berkeley Geochronology Center and now a research scientist at the University of Geneva; Marcia Ernesto, a paleomagnetist at the University of S“o Paulo; and geochemist Enzo Piccirillo of the University of Trieste.

In 1995, a group led by Renne attributed another mass extinction to a similar magma flood in Siberia 250 million years ago. A third mass extinction associated with the demise of the dinosaurs has been linked to a large volcanic flood that produced the Deccan Traps in India.

Though many basalt outcrops such as the well-studied Palisades sill had been dated to the general time period of the devastating eruption, no one had realized the extent of the eruptions until now.

May 5 - 11, 1999 (Volume 27, Number 33)
Copyright 1999, The Regents of the University of California.
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